Detec%ng the unseen through applica%on of a robust mark- resight design for es%ma%ng Indo- Pacific humpback dolphin demographics in Bangladesh

Transcription

1 Detec%ng the unseen through applica%on of a robust mark- resight design for es%ma%ng Indo- Pacific humpback dolphin demographics in Bangladesh Rubaiyat Mowgli Mansur, Samantha Strindberg, Brian D. Smith and Masudur Rahman

2 Es;ma;ng the demographic parameters of highly mobile wildlife is par;cularly challenging when detec;on is imperfect and only an unknown por;on of the geographical range of a super- popula;on can be sampled. This has vital implica;ons for protec;ng species whose distribu;on and conserva;on risk are poorly known. A mark resight analysis under a robust design was applied to Indo- Pacific humpback dolphins Sousa chinensis in open estuarine waters offshore of the Sundarbans mangrove forest, Bangladesh.

3 Ganges- Brahmaputra- Meghna Sundarbans Swatch- of- No- Ground

4 Ganges River dolphin Platanista gange9ca Finless Porpoise Neophocaena phocaenoides Irrawaddy dolphin Orcaella brevirostris Pantropical spoeed dolphin Stenella a4enuata Indo- Pacific humpback dolphin Sousa chinensis Spinner dolphin Stenella longirostris Indo- Pacific BoElenose dolphin Tursiops aduncus Bryde s whale Balaenoptera edeni False killer whale Pseudorca crassidens Rough- toothed dolphin Steno bredanensis Bangladesh is a global hotspot for cetacean diversity and abundance.

5 During three winter seasons in , 88 humpback dolphin sigh;ngs were made along more than 5,000 km of track line.

6 Almost 43,000 dorsal fin photographs resulted in the iden;fica;on of 405 individuals, with an average re- sigh;ng rate of 0.94, and abundance es;mates of 132, 131 and 635 for the three winter seasons, respec;vely.

7 One of the crucial piece of informa;on necessary to use a mark- resight model is knowledge of the propor;on of unmarked individuals present in each group. This is important because tradi;onal mark- recapture models only es;mate the popula;on of marked individuals.

8 A graph of the cumula;ve number of newly iden;fied dolphins versus the number of photo- iden;fica;on days indicates that the popula;on is probably larger than the actual number of iden;fica;ons. The stair- step pasern indicates that the increase is mainly due to the occurrence of large groups with a large propor;on of newly iden;fied individuals.

9 The considerable increase in the abundance es;mate during the third season can be explained by the large number of new dolphins iden;fied for the first ;me during the third year. This includes a single group of 205 photo- iden;fied individuals which is by far the largest group ever recorded of the species.

10 INDIA BANGLADESH What is clear from these es;mates, together with the high probability of individuals remaining in an unobservable state (55%), is that the individuals sampled in the study area are part of a larger superpopula;on using a more extensive geographic region. This conclusion is supported by the rela;vely low rate of re- iden;fica;ons (0.94/individual).

11 These findings have par;cular conserva;on relevance due to the high bycatch of humpback dolphins in gillnet fisheries. From the 405 Individuals in the photo iden;fica;on catalog, 15 % exhibited scars and mu;la;ons that were almost certainly associated with entanglements in fishing gears.

12 Also recent gene;c evidence indicates that the upper Bay of Bengal supports a gene;cally unique popula;on of humpback dolphins with no shared haplotypes among extensive samples analyzed from other members of the genus.

13 Conclusions A Pollock s robust design uses data collected from open and closed sampling periods to generate an es;mate of temporary emigra;on. This represents a significant advancement over tradi;onal open popula;on models which assume emigra;on is permanent. This has vital implica;ons for protec;ng species whose distribu;on and conserva;on risk are poorly known. The results of this study indicate that the coastal waters of Bangladesh support a por;on of a larger super- popula;on that occupies a more extensive area in both Bangladesh and India. This is among the largest popula;ons of humpback dolphins but currently threatened by high mortality from gillnet entanglement.

14 Thank you. WILDLIFE CONSERVATION SOCIETY Bangladesh Program